Colorants are widely used in a variety of applications to provide a useful and aesthetic appearance to articles and printed images. Standard colorants are most often classified as either pigments or dyes. For some applications, colorants are directly mixed into an article of manufacture. In other applications, ink compositions containing colorants are used for printing of color images. Such ink compositions typically include a liquid vehicle such as an appropriate solvent containing the dye or the pigment. Dye-based inks generally contain a colorant that is soluble in the given liquid vehicle, while pigmented inks contain an insoluble dispersed solid colorant to achieve color. Some colorants comprise both dyes and pigments.
Dye and pigment colorants each have certain properties that are beneficial in certain circumstances and which can provide predetermined color properties to a printed image. However, dyes and pigments also have various limitations and drawbacks which can limit the choice of dyes and pigments for particular applications. Dyes typically exhibit good chroma and long-term stability in solution, however, they tend to have poor waterfastness, lightfastness and smear resistance. In contrast, pigment colorants usually provide good waterfastness, good lightfastness and good smear resistance, but they generally have limited chroma and are typically insoluble in the liquid vehicle. The discrete pigment particles tend to clump or agglomerate if they are not stabilized in the vehicle. To prevent pigments from agglomerating or settling out of a liquid suspension, any of a number of approaches can be taken to stabilize the pigments in dispersed form.
With respect to ink-jet inks, stable pigment dispersions, dispersants or surfactants are customarily added to the ink vehicle. The dispersant is typically a polymer formed by polymerizing hydrophobic monomers, hydrophilic monomers, hydrophobic copolymers, and/or hydrophilic copolymers. The dispersant binds to a surface of the pigment, providing stability to the pigment dispersion. When printing is desired, the pigment is precipitated from the ink-jet ink and onto the print medium by solvent evaporation. This process may be enhanced by the presence of salts, acids, or polymers in the print medium, to provide printed images with improved durability and gloss. In many cases a large amount of the dispersant is needed to produce these desirable properties, which increases the viscosity of the ink, detrimentally affecting the firing reliability and print quality of ink-jet pens.
Another approach to stabilizing a pigment in an ink-jet ink involves modifying the surface of the pigment to include covalent attachment of polymeric or organic groups. The surface-modified pigments are referred to in the art as “self-dispersed pigments” or “polymer-attached pigments.” Using surface-modified pigments offers more latitude in ink formulation and reduces the viscosity of the ink compared to those that contain dispersants. The presence of any unbound polymeric or organic groups, however, would tend to detrimentally increase the viscosity of the ink. This, in turn, can lead to nozzle clogging in ink-jet pens.
Various polymer-dispersed pigments in which the pigment is encapsulated with a polymer having hydrophilic and hydrophobic moieties have been reported, some of which are stable in the liquid vehicle at a pH of from about 5.5 to about 8.5. Ink-jet inks including certain polymer-attached pigments with a variable particle size have been investigated. Polymeric colorants that include a pigment having a covalently attached polymer have been described in which a dye is covalently attached to the polymer. In some instances, a dispersant can be covalently attached to one or more of the pigment, the polymer and the dye. There is continuing interest in developing additional polymeric colorants and ink formulations, especially those which combine dye and pigment colorants.